Reproductive Hormonal Alterations in Obesity: The purpose of this proposal is to examine the mechanisms responsible for reduced central neural reproductive drive in obese women. Women who are obese will be compared to normal weight women and will receive GnRH, followed by a submaximal exposure to GnRH antagonist, to assess GnRH input. To test the hypothesis that obesity is associated with increased local aromatase, letrozole, a potent aromatase inhibitor, will be given to attempt to restore normal reproductive hormone secretion. Corpus luteum function will be examined in obese women, and a newly developed nonhuman primate model of obesity will allow direct examination of corpus leteum tissue.